1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the same, and more particularly to a gallium nitride-based semiconductor device and a method for manufacturing the same.
2. Related Art
Nitride-based III-V group compound semiconductors such as gallium nitride (GaN) have a wide band gap, and such a characteristic is utilized to research and develop high-intensity ultraviolet to blue/green light emitting diodes and blue-violet laser diodes. Further, high-frequency and high-power field-effect transistors using nitride-based III-V group compound semiconductors have been manufactured.
The layer structure of a III-V group compound semiconductor device is formed by crystal growth. In general, the surface of a semiconductor crystal is not directly exposed to air or the like, and is coated with, for example, a metal electrode or a protective film made of Al2O3 or SiO2. Such a protective film prevents the deterioration of device characteristics. For example, in the case of a laser diode, a cleaved surface is used as a reflector. If such a cleaved surface is not coated with some kind of a protective film, moisture, organic or inorganic matter, and the like contaminate the cleaved surface so that the refractive index thereof is changed, thereby causing fluctuations in reflectivity. Fluctuations in reflectivity change the characteristics of the laser diode, thus deteriorating device reliability. Therefore, based on expertise in laser diodes using GaAs-based or InGaAlP-based materials, an Al2O3 film has often been used as a film for protecting a cleaved surface (see, for example, K. Itaya et al.,“Effect of facet coating on the reliability of InGaAlP visible light laser diodes”, Applied Physics Letters, vol. 53, no. 15, pp. 1363-1365, Oct. 10, 1988).
The present inventors have manufactured a nitride-based III-V group compound semiconductor blue-violet laser diode using an Al2O3 film as a film for protecting a cleaved surface. However, the lifetime of the device was about 700 hours when measured under the conditions required for high-speed disc recording, that is, at an optical output of 60 mW in a continuous wave mode (or at an optical output of 120 mW in a pulse oscillation mode). Then, the end face of the device was observed with an electron microscope, and was found to be deteriorated. The reason for this can be considered as follows. The wavelength of blue-violet laser light is shorter than that of near-infrared laser light emitted from a GaAs-based laser diode or red laser light emitted from an InGaAlP-based laser diode, and therefore blue-violet laser light has higher photon energy, thereby causing damage to the protective film made of Al2O3.
Meanwhile, a method for prolonging the lifetime of a nitride semiconductor laser diode has been proposed (see, for example, Japanese Patent Laid-open Publication No. 2002-237648). In this method, an SiN film is formed on both resonator end faces of a nitride semiconductor laser diode so as to have a thickness of about 1 nm, and then a low-reflection coating of SiO2 is applied onto one resonator end face and a high-reflection coating of SiO2/TiO2 is applied onto the other resonator end face. According to this method, the lifetime of the nitride semiconductor laser diode measured at an optical output of 1 mW at an operating temperature of 50° C. is prolonged to several hundred hours. However, this method cannot prolong the lifetime of the nitride semiconductor laser diode at a high optical output of 60 mW in a continuous wave mode (or at an optical output of 120 mW in a pulse oscillation mode).
Further, there is known a method in which a single crystal film of AlxGa1−xN (0 ≦×≦1) is formed on a resonator end face or faces of a nitride semiconductor laser device (see, for example, WO 03/036771). However, according to this method, the semiconductor device is heated in an apparatus for forming a single crystal film before a single crystal film is formed on the end face of the device, and therefore the end face of the device is oxidized by oxygen remaining in the apparatus, thus deteriorating the end face of the nitride semiconductor device. In this case, even if a film is formed on such a deteriorated end face, it is impossible to improve the reliability of the semiconductor device.
As has been described above, according to the conventional technology, it is difficult to obtain a high-reliability semiconductor device comprising nitride-based III-V group compound semiconductor layers.